In downhole operations, such as in the oil and gas industry, underreaming operations are often required to increase or standardise the gauge of bores downhole. The drilled bores are lined with tubing, known as casing or liner, and cement is injected into the annulus between the casing and the surrounding bore wall. Typically, the bore is drilled in sections, and after drilling a section that section is lined with casing. Following cementing of the casing, the next section of bore is drilled. However, as the drill bit utilised to drill the next section must pass through the existing casing, the drill bit will of necessity be of smaller diameter than the drill bit used to drill the previous section. It is often considered desirable to enlarge the bore diameter below a section of casing beyond the drill bit diameter, and this is normally achieved by means of an under-reamer mounted above the drill bit.
Where a section of bore is drilled underneath a section of casing or cement sheath, the drill-bit may have a limited diameter such that the borehole drilled may be of a narrower gauge than the lined or cased section of the newly drilled bore section. Particularly in offshore and deepwater wells, getting the largest casing size possible into the ground is critical to ensure target depth (TD) can be reached with the largest bit size possible, thus maximising production and facilitating access. Under-reaming the pilot bore drilled by a typically-fixed diameter drill bit enables casing sizes to be maximised by providing sufficient open hole clearance to allow the maximum pass through casing size to be selected.
To reach the lower section to be drilled, the drillbit may need to pass through restrictions, including any casing, narrowings or sheaths above the section to be drilled. Reaming or underreaming the newly drilled section may allow the newly drilled bore section to have an increased diameter, possibly up to the same diameter as the upper section of bore. As with the drill bit, the reamer or underreamer may also need to pass through a restricted diameter such as any upper casing, sheath or liner. Accordingly, reamers are typically tripped-in in a retracted configuration with cutter blocks at a reduced diameter, with the cutters being extended when the reaming operation is to commence below any restrictions. Once reaming is completed, cutters are typically returned to the retracted position and the tool retrieved from the bore.
Examples of under-reamers are described in Applicant's U.S. patent application Ser. No. 13/198,594, published as US2012031673 (A1) and applicant's International patent applications, Publication No.s WO2007/017651 and WO2010/116152, the contents of each being incorporated herein by reference.